It is well known that freezing and thawing cycles can be extremely damaging to hardened cement compositions, such as concrete and that the best known technique to prevent or reduce the damage done is the incorporation in the composition of microscopically fine pores or voids. The pores or voids function as stress relievers and can therefore protect the concrete from frost damage. The method used in the prior art for artificially producing such voids in concrete has been by means of air-entraining agents, which stabilize tiny bubbles of air that are entrained in the concrete.
Air-entrained concrete is one of the most difficult types of concrete to make, as the content of air entrained in the concrete cannot be controlled by direct quantitative means, but only indirectly through the amount/type of air-entraining agent added to the mixture. Air content is also influenced by factors such as the composition and particle shape of the aggregates, the type and quantity of cement in the mix, the consistency of the concrete, the type of mixer used, the mixing time, and the temperature.
Those skilled in the art have learned to control these influences by the application of appropriate rules for making air-entraining concrete. They do, however, require the exercise of particular care in making such concrete and continually checking the air content, because if it is too low, the frost resistance of the concrete will be inadequate, while, on the other hand, if it is too high, it will adversely affect the strength.
However, additional difficulties arise in connection with cementitious compositions of stiff consistency such as that for making dry cast mixtures and articles. In this case, the air content cannot be tested in the fresh cementitious compositions, but only in the hardened cementitious compositions, which is too late for controlling the air content during mixing.
Cementitious dry cast mixtures are used to form many articles, for example, concrete pipe, roof tile, masonry units, paver units, extruded plank, and any other preformed cementitious articles, in a mold or from an extrusion die. Each of these applications has basic desired characteristics that are critical in terms of producing quality finished units.
Therefore, it is desirable to provide an admixture that produces a freeze-thaw durable void structure directly in a dry cast mixture without requiring the stabilization of air bubbles during mixing. The void structures may comprise optimally sized voids, which when incorporated into the dry cast mixture, will provide the cementitious composition with improved freeze-thaw durability. The admixture should also reduce or eliminate the loss of compressive strength for products manufactured from dry cast mixtures containing air-entraining chemical admixtures.
Using polymeric microspheres, it should be possible to make cementitious compositions with a substantially lower and less variable voids content and, therefore, with higher and more uniform strength than can be accomplished with an air-entraining agent.